swf: SWF parsing returns slices of input, use Windows-1252 string encoding in SWFv5

[Herschel]

Start some much needed spring cleaning of swf:

• Switch to bitstream_io for bit parsing. Add a reader.bits() that returns the bitstream reader on demand, which gives clear boundaries between byte-aligned and bit-packed structs.
• Change swf::Tag structs to return slices for raw data and strings instead of copying. This fixes remaining issues with out-of-bounds jumps in clip/button actions. (fixes Miami Shark - Start Game button doesn't do anything #1397, Mspacman keys not working #2442, SCANDAL: Bloons Tower Defense 1&2 not working #2618, ...)
• Add swf::SwfStr type to handle string encoding. SwfStr is a sequence of bytes (allowing for invalid encodings) and can be decoded based on the SWF version, using encoding_rs to decode when necessary. This fixes the parsing of various early SWFs with non-ASCII (fixes swf: Fall back to WINDOWS-1252 for text decoding #1499, Steppenwolf 4-3 cannot be parsed #1533, Error running definition tag: DefineEditText, got Invalid data #1903, Animation skips to the end and goes into a loop #2043, ...) and will also allow us to make the encoding configurable.
• Remove some extraneous Cursor from throughout the APIs.

TODO (probably future PRs):

• Add option for specifying encoding for SWFv5 files. (Currently defaults to WINDOWS-1252).
• Change the avm2 reader to return slices.
• Clean up the SWF testing and make it use a macro similar to the core Ruffle regressions tests.
• Use bitflags or for some of the dense bitflags in various swf structs. (Many of the structs have long lists of bools).
• Reintroduce parsing SWF header without decompressing the entire SWF. (I would like to return an "extended header" including FileAttributes info.)
• Reintroduce the convenience API of read_swf reading a sequence of owned tags.
• Change the parsing functions to free functions instead of having Reader struct. Ruffle core can use these free functions directly. Reader etc. could be reintroduced as part of the above convenience API in swf.
• Change AvmString representation to work similar to SwfString.

[Toad06]

Tested the PR on the web backend and it works perfectly, very impressed how much stuff this PR fixes. 👍

Just a little thing, it seems like every file always logs this warning:
SWF length doesn't match header, may be corrupt

A last thing, but possibly unrelated, this (not legit) file cannot be parsed because of unknown tags 903 and 1002 (I've never seen those values in any other files I've tested).

[Herschel]

Just a little thing, it seems like every file always logs this warning:
SWF length doesn't match header, may be corrupt

Fixed!

A last thing, but possibly unrelated, this (not legit) file cannot be parsed because of unknown tags 903 and 1002 (I've never seen those values in any other files I've tested).

Looks like the file is contains these bogus tags for whatever reason. (Ffdec shows the same tag #s).